A circulating fluidized bed combustor (CFBC) may be used in different applications, such as thermal power generation, waste incineration and steam generation. The fuels that are commonly employed in the circulating fluidized bed combustor include fossil fuels, such as coals and petroleum cokes. However, an exhaust gas generated from the combustion of these fuels in the circulating fluidized bed combustor normally includes sulfur-containing materials, such as sulfur dioxide, which may cause acid rain.
Removal of sulfur from the sulfur-containing materials in a CFBC may be achieved by limestone (mainly composed of calcium carbonate, CaCO3) that is directly injected into a furnace of the CFBC. Limestone can be thermally decomposed into quicklime (CaO) and carbon dioxide at around 600° C. Calcium oxide (CaO) reacts with sulfur dioxide and oxygen to produce calcium sulfate, thereby permitting desulphurization of the exhaust gas. The followings are reactions taken place during the CFBC in-situ desulphurization.CaCO3→CaO+CO2 CaO+½O2+SO2→CaSO4 
Combustion of sulfur-containing carbonaceous materials with the limestone by virtue of an in-situ desulfurization process normally generates a gaseous product, fly and bottom ashes. Those ashes are particles consisting of CaCO3, CaO, CaSO4, Ca(OH)2, etc. The formation of calcium sulfate (CaSO4) at the ash particle surface suppresses the core calcium oxide desulfurization reaction. Typically, the sulfurized calcium is less than 50%. Therefore, when fly ashes and bottom ashes are further converted to calcium sulfate dihydrate (gypsum, CaSO4.2H2O), a large amount of sulfuric acid is required.
As such, there is a need to further develop a CFBC desulfurization method for a sulfur-containing carbonaceous material which uses less amount of the sulfuric acid when the ashes are further converted to gypsum.